1. Technical Field
The present invention relates to an apparatus for producing U3O8 powder, and, more particularly, to an apparatus for producing U3O8 powder having a large specific surface area by efficiently discharging the heat occurring when UO2 sintered pellets are oxidized.
2. Description of the Related Art
UO2 sintered pellets, which are used as nuclear fuel, are produced by compacting or sintering UO2 powder prepared by a dry process or a wet process. Domestically-used UO2 sintered pellets have been being produced by UO2 powder prepared by a dry conversion process (hereinafter, referred to as “DC process”).
Since UO2 sintered pellets can more easily control the release of fission gas occurring during the burning of nuclear fuel in an atomic reactor as their grain size increases, they can increase the rate at which nuclear fuel is burned. The acceptable range of sintered density of UO2 sintered pellets in technical specifications is 94% to 96.5% of theoretical density (TD). In this case, in order to safely use the UO2 sintered pellets in an atomic reactor, the sintered density of the UO2 sintered pellets must be about 95% TD.
Therefore, conventionally, in order to make the sintered density of UO2 sintered pellets fall inside the acceptable range of the technical specifications, 2 wt % to 20 wt % of U3O8 powder, whose volume per unit weight is about 30% greater than that of UO2 powder, U3O8 is mixed with UO2 powder to form a mixed powder of UO2 and U3O8, and then the mixed powder is compression-molded and then sintered at a temperature of 1650° C. to 1800° C. under a hydrogen gas atmosphere to prepare UO2 sintered pellets.
The reason why U3O8 powder having a large specific surface area is mixed with UO2 powder is that U3O8 powder, having a large specific surface area, improves the sinterability of UO2 powder, so that UO2 sintered pellets having a large grain size can be easily prepared, and UO2 sintered pellets having high density and a stable porous structure can be obtained.
For this reason, conventionally, defective UO2 sintered pellets or UO2 powder occurring during the preparation of UO2 sintered pellets were compacted to make a slug as a preform, and then the slug was sintered to form UO2 sintered pellets, and then the UO2 sintered pellets were oxidized at a temperature of 300° C. to 400° C. for 10 hours to 24 hours to prepare U3O8 powder having a large specific surface area.
However, a conventional apparatus for producing U3O8 powder is designed such that the apparatus is suitable to oxidize an extremely small amount of UO2 sintered pellets (an amount that is small to such a degree that UO2 sintered pellets do not pile up in a conventional scrap box) under laboratory conditions, and is configured such that the apparatus is not provided with an additional device for dissipating the heat generated by the oxidization of UO2 sintered pellets. Therefore, the conventional apparatus for producing U3O8 powder is not influenced by the heat generated by the oxidization of UO2 sintered pellets when a small amount of UO2 sintered pellets is oxidized, but is problematic in that the heat generated by the oxidization of UO2 sintered pellets increases the temperature around the UO2 sintered pellets to 400° C. or more when the amount of UO2 sintered pellets is increased to about 10 kg or more.
For this reason, the conventional apparatus for producing U3O8 powder is problematic, in that the higher temperature occurring when a large amount of UO2 sintered pellets is oxidized influences adjacent UO2 powder and UO2 sintered pellets, so that UO2 sintered pellets are oxidized at a temperature (about 400° or more) higher than the desired temperature at which they should be controlled, thereby decreasing the specific surface area of the produced U3O8 powder.